The present invention relates generally to ceramic arc discharge lamps and more particularly to a cathode for a high watt ceramic metal halide lamp for use in combination with a non pulse-start ballast.
Discharge lamps produce light by ionizing a fill material, such as a mixture of metal halide and mercury in an inert gas, such as argon, with an arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge chamber, which maintains the pressure of the energized fill material and allows the emitted light to pass through. The fill material, also known as a xe2x80x9cdose,xe2x80x9d emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, including color temperatures, color rendering, and luminous efficiency.
Arc tube chambers composed of fused silica xe2x80x9cquartzxe2x80x9d are readily formed. However, the lifetime of such lamps is often limited by the loss of the metal portion of the metal halide fill (typically sodium) during lamp operation. Sodium ions diffuse through, or react with, the fused silica arc tube, resulting in a corresponding build-up of free halogen in the arc tube. Quartz arc tubes are relatively porous to sodium ions. During lamp operation, sodium passes from the hot plasma and through the arc tube wall to the cooler region between the arc tube and the outer jacket or envelope. The lost sodium is thus unavailable to the discharge and can no longer contribute its characteristic emission. The light output consequently diminishes and the color shifts from white toward blue. The arc becomes constricted and, particularly in a horizontally operated lamp, may bow against the arc tube wall and soften it. Also, loss of sodium causes the operating voltage of the lamp to increase and it may rise to the point where the arc can no longer be sustained, ending the life of the lamp.
Ceramic discharge lamp chambers were developed to operate at higher temperatures than quartz, i.e., above 950xc2x0 C., for improved color temperature, color rendering, and luminous efficacies, while significantly reducing reaction with the fill material. U.S. Pat. Nos. 5,424,609; 5,698,984; and 5,751,111 provide examples of such arc tubes. While quartz arc tubes are limited to operating temperatures of around 950xc2x0 C. to 1000xc2x0 C., due to reaction of the halide fill with the quartz, ceramic alumina arc tubes are able capable of withstanding operating temperatures of 1000xc2x0 C. to 1250xc2x0 C. or higher. The higher operating temperatures provide better color rendering and high lamp efficiencies. Ceramic arc tubes are less porous to sodium ions than quartz tubes and thus retain the metal within the lamp Various techniques are available for fabricating the arc tubes, including casting forging, machining, and various powder processing methods, such as powder injection molding (PIM). In powder processing, a ceramic powder, such as alumina, is supported by a carrier fluid, such as a water-based solution, mixture of organic liquids, or molten polymers. The mixture can be made to emulate a liquid, a plastic, or a rigid solid, by controlling the type and amount of carrier and the ambient conditions (e.g., temperature).
One problem with such lamps is that the light output (lumen maintenance) decreases with time. To reduce the rate at which the light output decreases, the argon pressure within the lamp is increased. Since the breakdown voltage (the voltage necessary to initiate an arc) generally increases with pressure, the higher internal pressures require higher voltages for initiating the arc. To initiate the formation of an arc, ceramic metal vapor lamps are conventionally fitted with a ballast having an igniter. The igniter senses that the arc has not formed and generates voltage pulses which cause breakdown of the vapor and permit current flow between the two electrodes. The igniter then turns off. Typically, high voltage pulses in the 4-5 kV range are used in 1-2 microsecond pulses. Thus, high wattage (over 175 watts) ceramic metal halide lamps have generally been limited to use in igniter started lamps (so-called Pulse Arc Ballasts). However, such lamps make up only a small proportion of the commercially-produced lamps available. The large majority (over 90%) of ballasts produced for high wattage metal halide lamps are not fitted with igniters Instead, the ballast is fitted with a constant wattage autotransformer (CWA) circuit
One way to initiate the discharge in a lamp without an igniter is to have a three electrode system The two main electrodes are spaced from each other by a suitable distance for maintaining an arc during operation of the lamp, typically 2-3 cm. The third electrode is closely spaced to one of the main electrodes, providing a much smaller gap which allows the breakdown to occur more readily. Once breakdown has occurred between the third electrode and the closely spaced main electrode, the voltage to the third electrode is switched off and the arc is readily generated between the two main electrodes. Ceramic discharge tubes, however, have not been amenable to 3 electrode systems. Quartz discharge tubes which use 3 electrode systems are formed by a quartz pinching process which cannot be used with the much more brittle ceramic material. Because ceramic material is brittle, new designs and processes are required to prevent cracking during discharge tube fabrication and lamp operation.
The present invention provides a ceramic discharge tube capable of accommodating three electrodes, which overcomes the above-referenced problems and others.
In an exemplary embodiment of the present invention, a discharge vessel is provided. The discharge vessel includes a body of a translucent ceramic material. The body includes a barrel portion and first and second end walls closing ends of the barrel portion to define an interior chamber. At least a first generally cylindrical tube extends from the first end wall and opens into the chamber A second generally cylindrical tube extends from the second end wall and opens into the chamber. The first and second cylindrical tubes each have a shoulder portion which provides a curved transition between the cylindrical tube and the end wall. A fill is contained in the body for creating a discharge. Electrodes are supported in the chamber. The electrodes include a first main electrode, a second main electrode, and an initiator electrode disposed a preselected distance from the first electrode. The first main electrode and the initiator electrode are electrically connected with first and second lead through disposed in at least the first cylindrical tube. The second main electrode is connected with a third lead through disposed in the second cylindrical tube.
In another exemplary embodiment of the present invention, a method of forming a discharge vessel is provided. The method includes forming a generally cylindrical barrel and first and second end plugs from a ceramic material The first and second end plugs each include an end wall and at least one tubular leg portion. At least one tubular leg portion is joined to the end wall by a shoulder portion which provides a curved transition between the leg portion and the end wall. The method further includes heating the barrel and the end plugs together to a sufficient temperature to join the end plugs to the barrel and form a translucent body thereof. Three electrodes are positioned in the body. Electrical connections are provided for the three electrodes through the leg members. A fill of a material suitable for creating a discharge is sealed in the body.
In another exemplary embodiment of the present invention, a method of forming a translucent ceramic arc tube is provided. The method includes forming a slurry of a ceramic powder and a liquid and introducing the slurry into a cavity of a mold. The mold is formed of a porous material which absorbs the liquid Liquid is absorbed from the slurry to form a body capable of being removed from the mold without breaking. The body is fired at a sufficient temperature to form an arc tube having a hollow barrel with leg members extending from the barrel. The leg members have a total of at least three bores for receiving electrical connections for first, second, and third electrodes therethrough
One advantage of at least one embodiment of the present invention is that a ceramic metal vapor lamp is provided which operates at high internal pressures without an igniter.
Another advantage of at least one embodiment of the present invention is the provision of a high strength ceramic discharge vessel which is resistant to fracturing.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.